1. Field of the Invention
The present invention relates to a tension detecting device which detects the tension of a belt such as, for example, a webbing belt structuring a seat belt device of a vehicle, or the like.
2. Description of the Related Art
Vehicles such as automobiles and the like are equipped with so-called seat belt devices which restrain, by an elongated, strip-shaped webbing belt, the body of a vehicle occupant who is seated in a seat. Further, vehicles are equipped with so-called air bag devices which, when the vehicle rapidly decelerates or the like, expand and unfold a bag body in front of the vehicle occupant seated in the seat, such that the bag body catches the body of the vehicle occupant which is attempting to move toward the front of the vehicle due to inertia.
Such seat belt devices are not only for use in restraining the body of a vehicle occupant. For example, there are cases in which the seat belt device is used to fix a child seat which is set on a seat and which is used for the seating of a child passenger.
When a child seat is fixed on a seat, it is preferable for the air bag device corresponding to the seat where the child seat is fixed to not deploy. Thus, usually, a switch for prohibiting deployment of an air bag device is provided at the vehicle. When a child seat is fixed to a seat, the driver operates the switch so as to prohibit deployment of the air bag device. However, in a structure having such a switch which is operated manually, the switch must be operated each time the child seat is installed or removed, and operation of the switch is bothersome.
Thus, detecting devices for automatically detecting the absence/presence of installation of a child seat and prohibiting deployment of the air bag device have been desired. A seat belt tension measuring system (hereinafter called “tension detecting device”) is known which, on the basis of the tension applied to the webbing belt, detects whether or not a child seat is installed. (See, for example, US Patent Publication of Application No. 2002/0043795A1.)
Namely, in a seat belt device, in order to alleviate the constrained sensation of the vehicle occupant in the usual state in which the webbing is applied to him/her, the webbing loosely restrains the vehicle occupant, and the tension applied to the webbing belt is small. On the other hand, when a child seat is installed, in order to impede movement of the child seat itself due to the vehicle accelerating, the seat belt device strongly holds and restrains the child seat, and the tension applied to the webbing belt is great.
The tension detecting device disclosed in US Patent Publication of Application No. 2002/0043795A1 is applied to the above-described seat belt device. In order to detect the tension applied to the webbing belt, the tension detecting device is structured such that a strain gauge is mounted to a metal plate. One end portion of the metal plate is fixed to a structural member of the vehicle, whereas tension of the webbing belt is applied to the other end portion of the metal plate. The strain gauge is a full-bridge strain gauge having four grids. In accordance with the extension of the metal plate in the direction of tension due to the tension and the contraction in the direction orthogonal to the direction of tension, the strain gauge outputs a signal corresponding to the tension applied to the metal plate, i.e., to the webbing belt (namely, the strain gauge measures or detects the tension).
The tension detecting device outputs the signal corresponding to the tension to a control unit of the air bag device. The control unit prohibits operation of the air bag device when the signal (the tension of the webbing belt) exceeds a threshold value set in advance, i.e., when a tension value corresponding to a state in which a child seat is installed is inputted.
However, in a conventional tension detecting device such as that described above, the tension is detected on the basis of the extension of the metal plate in the direction of the tension (longitudinal strain) and the contraction in the direction orthogonal to the direction of the tension (lateral strain). In other words, the lateral strain which is smaller (determined by Poisson's ratio) than the longitudinal strain is used to detect the tension. Therefore, the difference in the outputs of the extension and the contraction (the difference in the amplitudes of the changes of the electrical resistance values) is small, and a problem arises in that the tension detecting sensitivity is poor.